Situational awareness for the pilot of a vehicle includes timely knowledge of other vehicles and obstacles for safe operation of the vehicle. For example, aircraft pilots require situational awareness for safe navigation. Air traffic controllers also require situational awareness for accurate analysis of traffic conditions.
From the point of view of an aircraft pilot, as technology in air transportation is evolving and air traffic is dramatically increasing, the demands on the members of the flight deck are also increasing. To avoid flight path conflicts, the flight deck crew monitors considerable aircraft status information for multiple surrounding aircraft. Higher aircraft speeds magnify the burden by reducing the time in which the flight deck crew can respond to threatening situations. To assist the flight deck crew and enhance safety, several systems have been developed. One such system is based on transponders (e.g., MODE S, MODE C, MODE A), each hosted on a respective aircraft, that each report host aircraft identity and may report host aircraft altitude and other flight parameters. Typically, a transponder aboard a host aircraft replies to interrogations from other aircraft or ground based systems. A conventional collision avoidance system receives signals sent by transponders and uses these signals to identify the position of other aircraft. Results are provided as displayed data and possibly traffic advisories. Potential collision situations are identified. Resolution advisories may be provided suggesting an action to avoid the collision situation.
An aircraft collision avoidance system typically includes a directional antenna. The collision avoidance system uses signals from the directional antenna to determine the bearing from the host aircraft to a target (e.g., another aircraft). Bearing is displayed to the flight deck crew to assist them in obtaining visual contact with the target.
Aircraft collision avoidance systems may determine the bearing to a target by comparing magnitudes of signals received from a directional antenna of the type disclosed in U.S. Pat. No. 5,191,349 to Dinsmore, hereby incorporated by reference. Systems using this approach are generally referred to as amplitude monopulse systems and may be of the type disclosed in U.S. Pat. No. 6,329,947 to Smith, hereby incorporated by reference.
FIG. 1 illustrates the radiation pattern of a conventional directional antenna of the type used in an amplitude monopulse system. The antenna has four elements. The signals illustrated were measured on a four foot diameter flat ground plane. This radiation pattern is desired for performance of the antenna on all aircraft. As shown, the performance of the antenna in each of the four quadrants representing aft 100, port 110, fore 120, and starboard 130 is virtually identical. To determine the bearing of a target, a conventional collision avoidance system may use a model based on the radiation pattern of FIG. 1. When a target is detected, the bearing of the target is calculated by determining which beam 100, 110, 120, or 130 has the largest amplitude, determining which beam has the second largest amplitude, and taking the difference between the two. Based on this difference and the model, a bearing is determined.
Other conventional collision avoidance systems determine bearing to a target by determining and processing the phase angle of signals received at various elements of a directional antenna. Systems using this approach are generally referred to as phase monopulse systems.
Various factors, however, may degrade the accuracy of bearing determinations. For example, a collision avoidance system hosted on an aircraft having a fuselage with a relatively small radius of curvature may determine less accurate bearing because the ground plane assumed in the model differs from the surface of the fuselage. The degradation in accuracy tends to be more pronounced in an aircraft with a smaller fuselage than in an aircraft with a larger fuselage. The magnitude of the error differs: (a) with the elevation angle of the target; (b) with equipment (e.g., manufacturing variations from model conditions); and (c) with installation (e.g., reflections from adjacent antennas, wings, engines, and other aircraft obstructions).
In the patent to Smith a solution is presented to correct bearing errors solely related to effects due to fuselage curvature. These errors are predictable on the basis of fuselage design. Corrections may be generally applied to all aircraft having similar fuselage curvature. The techniques disclosed by Smith are difficult to apply for errors from other sources discussed above. Smith teaches manually setting a bearing indicator based on aircraft curvature. This technique leads to errors in determined bearing when a wrong value is set. A predefined value may be inappropriate in important cases. Smith uses fixed correction models that may no longer apply after future changes to an installation.
Without systems and methods of the present invention, further reduction of bearing errors cannot be obtained. Consequently, advisories and displays for systems such as a conventional traffic alert and collision avoidance system may be considered unreliable or lead to tragic loss of life and property.